Polar co-ordinate digitizers are useful for digitizing two dimensional forms because of their simplicity. The form is traced by a stylus which is connected by a cord to a relatively fixed base support at the origin axis of the polar coordinate system. The stylus is extensible from the base support to provide the length parameter, and a cord guide through which the cord extends and is pivoted at the origin and movable by lateral pressure of the cord provides the angle parameter. Such a system is disclosed in U.S. Pat. Nos. 3,832,781 and 4,500,749.
In the system as above, features such as accuracy, simplicity and small size are all desirable. For example, the lateral pressure of the cord provides the angle parameter and thus it is desirable to minimize this pressure for the sake of accuracy.